Method of combining images in a wearable display system

ABSTRACT

A wearable display system, such as a Head Mounted Display, having a display engine producing light, preferably linearly polarized light, which defines a synthetic image that is relayed to a wire grid polarizing combiner which overlays the synthetic image onto a real image of an object of the outside world, and wherein the real image is contemporaneously viewed through the wire grid polarizing combiner by the wearer of the system. The wire grid polarizing combiner can be curved in at least one axis, and preferably two axis such that optical power is added to the wire grid polarizing combiner.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/608,406, filed Sep. 8, 2004.

FIELD OF THE INVENTION

The invention relates to wearable display systems, such as a HeadMounted Display, and more particularly to a Head Mounted Display havingwire grid polarizing combiner which combines a synthetic image with acontemporaneously viewed real image.

BACKGROUND OF THE INVENTION

Wearable display systems, and particularly Head Mounted Display (HMD)systems, are well known in the art. HMDs have been conceptualized andmanufactured for over 40 years. The first HMD was constructed in the1960's by Ivan Sutherland. Since Sutherland's 40 pound monochrome HMD,great strides have been made in the field. Most modern augmented-visionHMDs use three general techniques for overlaying information onto anoutside real-world environment (augmenting reality or vision). The firsttechnique combines visual data with the outside world utilizing ametalized glass “combiner” optic. This combiner has a very thin layer ofmetal (usually aluminum), placed over a piece of glass to create apartially slivered mirror. An image is then reflected off this partialmirror and into the users visual field of view. Because of the nature ofthe mirror, the user is still able to view the outside environment,although somewhat attenuated. This mirror can be curved or flatdepending on the particular optical design.

Metalized combiners are among the most robust and versatile combiners.They offer uniform broadband reflectance allowing full color augmentedvision. However, they are limited by the very basic physics ofreflection and transmission. The portion of transmitted see-throughlight and the portion of reflected display light must equal 100%.Therefore, if 99% of the light from the outside world reaches the user,only 1% of the total display light will be reflected towards the user.This necessitates the use of a very bright and power hungry display.This general principle makes metalized combiner HMDs impractical formany demanding applications where lightweight, low power devices arerequired.

Other combining techniques include thin film coatings and HolographicOptical Elements (HOEs). Both of these techniques operate on theprinciple of reflecting very narrow and specific colors of light to theuser's visual field of view. In an optical design these combinersfunction in a similar manner as metalized combiners. These selectivewavelength combiners take a monochromatic light source and direct it tothe users visual field of view while letting other colors from theoutside environment pass onto the user. If full color augmentation isdesired, then three layers of either thin film stacks or three layers ofHOEs are required.

Most Military grade HMDs use a thin film coating type of combiner. Thistype of combiner offers a high degree of see-through vision with ahighly reflective monochrome display. The main disadvantage of this typeof combiner is that it only works well with narrow band sources, e.g.,lasers, or green phosphor CRTs. If full color augmented vision isrequired, then three narrow band laser sources are needed. These aretypically expensive and consume a lot of power. Moreover, even withexpensive narrow band reflective coatings, there will still be anoticeable loss of see-through light. There are also other practicalconsiderations such as only having specific angles of incidence overwhich the coatings will function properly, sometimes as small as +/−5degrees.

Holographic combiners are plagued by many of the same disadvantages asthin film coatings. Besides being extremely sensitive to color anddisplay light angle of incidents, holographic combiners can also beextremely susceptible to large changes in temperature.

Wire-grid polarizers are also well known in the art. The fist wire-gridpolarizer was developed in 1960 by George R. Bird and Maxfield Parrish.However, these polarizers were only for infrared wavelengths. Morerecently, others have pioneered the use of wire-grid polarizers in thevisible spectrum. Wire-grid polarizers are optical elements which workon the principle of transmitting and reflecting linearly polarized lightbased on its orientation to the wire-grid. Linearly polarized lightwhich is perpendicular to the wire-grid is passed. Linearly polarizedlight which is parallel to the wire-grid is reflected. These polarizerscan reflect over 90% of the polarized light over a large visiblespectrum and with angles of incidence greater than +/−20 degrees. Onecompany which presently makes wire grid polarizers for uses in thevisible spectrum is Moxtek, Inc., which is also the assignee of U.S.Pat. No. 6,208,463, for a Polarizer Apparatus for Producing a GenerallyPolarized Beam of Light.

Liquid Crystal on Silicon (LCOS) displays are known in the art, and workby using both pixel by pixel polarization modulation and fieldsequential color modulation. LCoS displays require linearly polarizedlight input. Conventionally, wire-grid polarizers have been used withLCoS displays to linearly polarize light before interaction with theLCoS display. It is also known for wire grid polarizers to occasionallybe used as a contrast enhancing, or “clean up,” polarizer located afterthe LCoS display, prior to projecting or viewing the image.

SUMMARY

According to the invention, a wearable display system, such as an HMD,is provided for augmenting a contemporaneously viewed “real image” of anobject in the outside world with a synthetic image using a wire gridpolarizing combiner. One embodiment the HMD can comprise a displayengine which produces linearly polarized light defining the syntheticimage, a light engine which produces light to illuminate the displayengine, a polarizing beamsplitter in the path of the light produced bythe light engine, wherein the polarizing beamsplitter polarizes at leasta portion of the light from the light engine and reflects the polarizedportion of light onto the display engine, enabling the display engine toproduce the linearly polarized light defining the synthetic image,display optics in the path of the linearly polarized light defining thesynthetic image, and a wire grid polarizing combiner through which thereal image is contemporaneously viewable. The display optics relay thelinearly polarized light to the wire grid polarizing combiner, and thewire grid polarizing combiner reflects the linearly polarized light suchthat the synthetic image is overlaid onto the contemporaneously viewablereal image. Illumination optics can also be provided to concentrate anddirect the unpolarized light from the light engine to the polarizingbeamsplitter which reflects the polarized light onto the display enginewhich produces polarized, spatially and temporally modulated lightrepresentative of the synthetic image, which is relayed by the displayoptics to the wire grid polarizing combiner where it is reflected towardthe pupil of the eye of the wearer of the HMD. The wire-grid polarizingcombiner reflects the synthetic image into the eye while allowing lightand images from the outside world, i.e., real images, to also becontemporaneously imaged by the eye thereby “combining” (basicallyoverlaying) the synthetic and real images.

The wire grid polarizing combiner can be curved in at least one axis,and preferably in two axes such that optical power is added to the wiregrid polarizing combiner. The dual axis curved wire grid combiner canhave the shape of a concentric shell section of a toroid. The dual axiscurved combiner can offer advantages over a flat combiner. One advantageof the curved combiner embodiment is that the powered optical element isclose to the eye enabling dramatically larger fields of view. Anotheradvantage is that the curved combiner may also result in concentratingmore light at the pupil of the user, creating a brighter image. Where adual axis curved wire grid polarizing combiner is utilized,complimentary designed display optics can be provided which includeimage aberration correction optics to correct image aberrations whichresult from the dual axis curvature of the combiner.

Another embodiment of the HMD can employ a display engine which includedintegral illuminate, and thus does not require the light engine orillumination optics. Linearly polarized light is not required for thewire grid polarizing combiner to function, but is preferable to avoidtransmission of light by the wire grid polarizer which could make theHMD undesirably visible. Thus, display engines which produce linearlypolarized light could be used which further obviates the need for apolarizer, making the polarizing beamsplitter unnecessary. On the otherhand, display engines which have integral illumination but do notproduce linearly polarized light could be used alone, but wouldpreferably be used in combination with a polarizer to linearly polarizethe light produced thereby such that only linearly polarized light ispresented to the wire grid polarizing combiner.

Further details, objects, and advantages of the invention will becomeapparent from the following detailed description and the accompanyingdrawings figures of certain embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawing figures, wherein:

FIG. 1 is a diagram of an embodiment of a wearable display systemaccording to the invention.

FIG. 2 is a diagram of an embodiment of illumination optics according tothe invention.

FIG. 3 is a diagram of an alternative embodiment of a wearable displaysystem according to the invention.

FIG. 4 is a diagram of another alternative embodiment of a wearabledisplay system according to the invention.

FIG. 5 is a diagram of a further alternative embodiment of a wearabledisplay system according to the invention.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Referring now to the drawing figures, wherein like elements are denotedby like reference numbers, there is illustrated in FIG. 1 an embodimentof a wearable display system, such as a Head Mounted Display (HMD) 10,which combines a synthetic image with a contemporaneously viewable “realimage” of the outside world. As shown, the HMD 10 can comprise a displayengine 12 which produces linearly polarized light defining the syntheticimage, a light engine 14 which produces light (typically unpolarized) toilluminate the display engine 12, a polarizing beamsplitter 16 in thepath of the light produced by the light engine 14 which polarizes atleast a portion of the light from the light engine 14 and reflects thepolarized portion of light onto the display engine 12 (enabling thedisplay engine 12 to produce the linearly polarized light defining thesynthetic image), display optics 18 in the path of the linearlypolarized light defining the synthetic image, and a wire grid polarizingcombiner 20 through which the real image 22 is contemporaneouslyviewable. The display optics 18 relay the linearly polarized light tothe wire grid polarizing combiner 20, and the wire grid polarizingcombiner 20 reflects the linearly polarized light to the eye 24 of theuser, such that the synthetic image is overlaid onto thecontemporaneously viewable real image of object 22 in the outside world.In the context of this disclosure, the “real image” refers to an imageof objects, e.g., object 22, in the “outside world,” as viewed by thewearer of the HMD 10 through the polarizing combiner 20. The displayoptics 18, also sometimes referred to as an eyepiece, typically includeimage enlargement optics which magnify the synthetic image produced bythe display engine 12. One such eyepiece, a magnifier, which can be usedis commonly referred to as an apochromatic eyepiece. The display optics18 can generally be complicated, compact versions of a simple magnifier.The display optics 18 are a type of magnifier (eyepiece) typicallydesigned for color correction and shorter focal lengths.

Illumination optics 26 can be provided to concentrate and direct theunpolarized light from the light engine 12 to the polarizingbeamsplitter 16. The light polarized by the polarizing beamsplitter 16is reflected onto the display engine 12 to produce polarized, spatiallyand temporally modulated light which is representative of (defines) thesynthetic image which is relayed by the display optics 18 into the wiregrid polarizing combiner 20 which reflects the image toward the pupil 25of the eye 24 of the wearer of the HMD. The wire-grid polarizingcombiner 20 reflects the synthetic image into the eye 24 while alsoallowing light and images from the outside world, i.e., real images 22,to be contemporaneously imaged by the eye 24 thereby “combining”(basically overlaying) the synthetic image onto the real image 22.

The display engine 12 can preferably be an LCoS display, although otherdisplay engines, producing either polarized or unpolarized light mayalso be used. In various embodiments of the HMDs described hereinafter,the display engine 12 can alternatively be, for example, LCD, OLED, andMEMS displays. However, an LCoS display is presently preferred as itproduces and reflects a full color, linearly polarized image. Accordingto the invention, the wire-grid polarizing combiner 20 is utilized as alight combiner, i.e.; combining the light from an outside image (thereal image 22) with the LCoS display light (the virtual image) toproduce a full color augmented display with the virtual image overlaidon the real image 22. When the wire grid polarizer 20 is mountedexternal to the other optics the users' natural full field of view isnot impinged upon. This allows the use of the HMD 10 in demandingtactical environments where situational awareness of the real world isvastly more important than overlaid synthetic information.

The LCoS display requires linearly polarized light input, which isspatially and temporally modulated by the LCoS display to produce thelinearly polarized light which defines the synthetic image. Thus, thelight engine 14 can be necessary to provide light to illuminate thedisplay engine 12, and the polarizing beamsplitter 16 can be necessaryto linearly polarize the light from the light engine 14 and reflect thepolarized light onto the display engine 12. The polarizing beamsplitterconverts about 50% of the unpolarized light to linearly polarized light.The polarized, spatially and temporally modulated light produced by thedisplay engine, e.g., the LCoS display, can then be relayed to the wiregrid polarizing combiner 20 by virtually any appropriately designeddisplay optics, such as display optics 18. The polarizing beamsplitter16 can preferably be a wire grid polarizing beamsplitter. Wire gridpolarizers have nearly perfect broad spectrum transmission (98%) andlarge angles of incidence (+/−20 degrees). This is achieved whilereflecting over 90% of the linearly polarized light from the display tothe user. Because the wire grid polarizing combiner 20 depends onpolarization for its transmission and reflection characteristics, andnot very narrow bands of light, broad sources such as LEDs can be used.This gives the advantage of not only having a bright overlay image, butalso much less power consumption.

The linearly polarized light from the display engine 12 is preferablycolored light, such as the modulated field sequential colored lightproduced by the LCoS. However, the linearly polarized light could alsobe monochrome. The light engine 14 can include both coherent andincoherent sources, such as, for example, LEDs, a laser source, anincandescent source, or a fluorescent source. If incandescent orfluorescent light sources are used, a color wheel can be necessary toproduce field sequential colored light.

The wire grid polarizing combiner 20 is essentially transparent to theuser, in that the wearer of the system can view real images of, forexample, an object 22, of the outside world by looking through the wiregrid polarizing combiner 20. However, the wire grid polarizing combiner20 substantially entirely reflects linearly polarized light relayedthrough the display optics 18 which is representative of the syntheticimage. Substantially all of the linearly polarized light is reflected bythe wire grid polarizing combiner 20, resulting in displaying a bright,high quality synthetic image to the eye 24 of the wearer. This syntheticimage basically augments the real image, e.g., of object 22, in that thesynthetic image reflected by the wire grid polarizing combiner 20appears to overlay the contemporaneously viewed real image of object 22of the outside world. Thus, as real images are being viewed through thewire grid polarizing combiner 20, the synthetic image augments the realimages. The wire grid polarizing combiner 20 can be flat, curved in asingle axis, or curved in more than one axis. As will be described inmore detail hereinafter, a preferred embodiment of the wire gridpolarizing combiner 22 can be curved in two axis, having the shape of aconcentric shell section of a toroid.

Referring more particularly to FIG. 2, in order to direct andconcentrate the light from the light engine 14 on the display engine 12,or more particularly the polarizing beamsplitter 16, illumination optics26 are provided in the light path between the light engine 14 and thepolarizing beamsplitter 16. As shown, a preferred embodiment of theillumination optics 26 can direct and concentrate light from the lightengine 14 onto the polarizing beamsplitter 16. To concentrate the light,the illumination 26 optics can comprise a total internal reflection(TIR) block which gathers and concentrates the light from the lightengine 14 onto the polarizing beamsplitter 16. The TIR block can have areflective, e.g., mirrored, off-axis parabola portion 28 positionedopposite the light engine 14, and a reflective TIR surface 30 oppositethe reflective off-axis parabola portion 28. Light from the light engine14 impinges on the mirrored off-axis parabola portion 28 whichconcentrates and reflects the light onto the opposite TIR surface 30.The opposite TIR surface 30 then totally internally reflects theconcentrated light onto the polarizing beamsplitter 16, which linearlypolarizes at least a portion (about 50%) of the light and reflects suchpolarized portion onto the display engine 12. In this way, substantiallythe total amount of light from the light engine 14 is internallyreflected onto the polarizing beamsplitter 16.

Referring now to FIG. 3, an alternative embodiment of a wearable displaysystem, such as a HMD 40, is illustrated which includes the displayengine 12, light engine 14, and illumination optics 26 described inconnection with the HMD 10 shown in FIG. 1. However, the HMD 40 canfurther comprise a curved wire grid polarizing combiner 42, preferably adual axis curved combiner, and different display optics 44 speciallydesigned for use with the dual axis curved combiner 42.

The dual axis curved combiner 42 can have important advantages,including enablement of a dramatically larger field of view, as much as65 percent larger than with a flat wire grid polarizing combiner.Another advantage is that the dual axes curvature can also result in ahigher degree of light being concentrated on the pupil 25 of the eye 24of the user, thus providing improved image brightness.

In general, display optics are employed to magnify the image produced bya display engine. This could be as simple as placing a single positivelens over the display, such as a liquid crystal display. Imagemagnification and image “quality” are the two main factors by which HMDoptics are judged. In the context of HMDs, the best definition ofmagnification (M) is the ratio of the angle (α1) between the unaided eyeand the display, and the angle (α2) between the aided eye and the imageproduced by the display, or M=α2/α1. This is also equal to the eyes“near distance” (254 mm) divided by the lens focal length. The angle α2is also referred to as the field of view of the HMD. The more“immersive” the HMD, the larger the field of view. One well knownlimitation as to how large α2 can be is called the “eye relief,” whichis a function of the size of the lens and the distance between the eyeand the last optical element. In an occluded HMD this is not as much ofan issue because the user only needs to look at the lens, and notthrough a combiner first, as with an augmented HMD. In an augmented HMD,i.e., where a synthetic image augments (is overlaid onto) the realimage, such as, for example, the HMD 10 illustrated in FIG. 1, the usermust look through the wire grid polarizing combiner 12 and then to theimage enlargement optics (display optics 18), i.e., the magnifier. Inthis type of configuration, the potential field of view will necessarilybe limited, that is unless the first component in the magnifier is thewire grid polarizing combiner itself, and thus the first display opticselement. Thus, by curving the wire grid polarizing combiner 42 asdescribed, optical power has been added to the polarizing combiner 42itself, such that the polarizing combiner 42 becomes a magnifier, andparticularly, the first component of the magnifier.

The addition of optical power to the polarizing combiner 42, and sincethe polarizing combiner 42 is positioned immediately adjacent the eye 24of the user (no other optical elements are between the user's eye andthe combiner), dramatically larger fields of view are enabled. In orderto be able to position such a large optically powered element so closeto the eye 24 of the user, a reflective off-axis element can benecessary, i.e., the dual axis curvature. In a preferred embodiment, theoptically powered dual axis wire grid polarizing combiner 42 can be inthe shape of a concentric shell section of a toroid, having a conicsurface profile known in the art as a bi-conic surface. This bi-conicsurface can be defined by an X radius of −40 mm with a conic constant of0.26, and a Y radius of −60 mm with a conic constant of 0.18. Theconcentric shell section is a 48 mm (x) by 28 mm (y) section, and is 23mm off-axis in (y). A result of the dual axis curvature as described isthat, as one skilled in the art will understand, such an off-axissection will result in the combiner 42 producing significant amounts ofimage aberrations which will need to be corrected by other opticalelements.

Accordingly, to accommodate the dual axis curved combiner 42, speciallydesigned display optics 44 can be provided. In particular, the displayoptics 44 can comprise image aberration correction optics designed to beused specifically with the dual axis curved combiner 42. Unlike theprevious embodiment of the HMD 10, the image enlargement function isperformed by the dual axis curved combiner 42 and the optics 44. Theoptics 44 are necessary to correct the image aberrations referred toabove which are caused by the dual axis curvature of the wire gridpolarizing combiner 42.

Referring still to FIG. 3, and as also shown in FIG. 4, the imageaberration correction is accomplished by overall lens grouping 44 (thedisplay optics), which is comprised of tilted and decentered lenses 48and rotational symmetric aspherical lenses 46. Working from the displayengine 12 toward the curved combiner 42, the synthetic image produced bythe display engine 12 is relayed by lens sub-group 46 into lenssub-group 48. Lens sub-group 48 “pre-distorts” the synthetic image whichis presented to the curved combiner 42. The curved combiner 42, incollaboration with overall lens grouping 44, provides the user with awell corrected, highly magnified image from the display engine 12 over alarge field of view. The optical power and aberrations created by thedual axis curved combiner 42 are thus an integral part of the opticaldesign of the HMD 40, and particularly the image aberration correctionlens group 44. Together the dual axis curved combiner 42 and imageaberration correction lens group 44 form high magnification displayoptics.

The light rays denoted by arrows 50 a, 50 b, 50 c represent differentfield angles which “see” different parts of the display. As with thepolarizing wire grid combiner 20 in FIG. 1, only linearly polarizedlight parallel to the wire grid is reflected, thus allowing the randomlypolarized light from the outside environment, i.e., the real image 22,to be seen by the user through the combiner 42, thus providing“augmented” vision.

One skilled in the art will realize that lens position and form areoptimized around standard image quality merits, such as, for example,Seidel aberration content over the visual spectrum and the opticalsystem modulation transfer function over field and wavelength.Conventional optics design software is also available to assist in thedesign of optics such as used in HMDs. One example of such software isZEMAX™, which is available from Zemax Development Corporation.

In FIG. 4, a further embodiment of an HMD 60 is illustrated which canbasically be include the same components as the HMD 40 in FIG. 3, minusthe light engine 14, illumination optics 26 and the polarizingbeamsplitter 16. In the HMD 60, light produced by the display engine 12(defining the synthetic image) is relayed to the curved wire gridpolarizing combiner 42 by display optics. The wire grid polarizingcombiner 42 is preferably curved in two axis, having the shape of aconcentric shell section of a toroid as described previously. Thedisplay optics 44 can also be the same as for the HMD 40 in FIG. 3,including being comprised of image aberration correction optics tocorrect for image aberrations caused by the off-axis section of the dualaxis curved combiner 42. The image aberration correction optics cancomprise the same overall lens group 44, which includes tilted anddecentered lenses 48 and rotational symmetric aspherical lenses 46.

The light produced by the display engine 12 which defines the syntheticimage does not have to be linearly polarized light. However, as will beexplained below, presenting linearly polarized light to the curved wiregrid polarizing combiner 42 can be preferred. A back illuminated LCDdisplay could be used with the HMD 60, since this type of display engineproduces linearly polarized light and does not require a separate lightengine. As suggested by the name, the back illuminated LCD displayincludes an integral light source. Examples of such LCD displays includethe displays on cell phones and digital cameras.

Another type of display engine which does not require a separate lightengine is for example, an organic light emitting diode (OLED) display.However, an OLED display generally does not produce linearly polarizedlight. As stated above, linearly polarized light is not required for theHMD 60 to function, but it can be preferable to present the wire gridpolarizing combiner 42 with linearly polarized light. A wire gridpolarizer reflects and transmits linearly polarized light regardless ofthe light which is presented to it. If a wire grid polarizer ispresented with linearly polarized light having an electric fieldperpendicular to the wires (the transmission axis) that make up the wiregrid, then substantially all of the light will be transmitted, subjectto the efficiency limits of the wire grid polarizer. On the other hand,if the electric field of the light presented to the wire grid isparallel to the wires, then substantially all of the light is reflected,likewise subject to the efficiency limits of the wire grid polarizer. Ifany other polarization state, including random, is presented to the wiregrid then that electric field is resolved or decomposed into its twoorthogonal components, and the magnitude of each component is eitherreflected or transmitted accordingly. If the most efficiency is desired,linearly polarized light is presented to the combiner. Otherwise, someportion of the light will be reflected and some portion will betransmitted (creating visibility issues).

Therefore, if the display engine 12 does not produce linearly polarizedlight, and it is desired that the wire grid polarizing combiner 42 doesnot transmit any light (so as to provide low visibility), a polarizer 62can be provided intermediate the display engine 12 and the displayoptics 44. The polarizer 62, shown in dashed lines in the figure (sinceit is optional), linearly polarizes the light from the display engine 12so that the wire grid polarizing combiner 42 reflects substantially allof the light representing the synthetic image, and essentially no lightrepresenting the synthetic image is transmitted through the combiner 42so as to give away the position of the wearer of the HMD 60.

The polarizer 62 need not be a beamsplitter, but it is preferably a wiregrid polarizer. For example, if the display engine were a (MEMSmicroelectro-mechanical systems) display which does not produce linearlypolarized light, a polarizer 62 could be used which was not abeamsplitter. However, a MEMS display does require a light engine. Thus,the HMD 60 would further require the addition of a light engine andassociated illumination options.

A further alternative embodiment of an HMD 70 according to the inventionis illustrated in FIG. 5, and can function essentially the same as theHMD 10 shown in FIG. 1. The HMD 70 similarly comprises a display engine12 which produces linearly polarized light defining the synthetic image,a light engine 14 which produces light to illuminate the display engine12, a polarizing beamsplitter 16 in the path of the light produced bythe light engine 14 which polarizes at least a portion of the light fromthe light engine 14 and reflects the polarized portion of light onto thedisplay engine 12, display optics 72 in the path of the linearlypolarized light defining the synthetic image, and a wire grid polarizingcombiner 20 through which the real image 22 is contemporaneouslyviewable. Display optics 72 relay the linearly polarized light to thewire grid polarizing combiner 20, and the wire grid polarizing combiner20 reflects the linearly polarized light as described previously, suchthat the synthetic image is overlaid onto the contemporaneously viewablereal image 22.

Illumination optics 74 can also be provided to concentrate and directthe unpolarized light from the light engine 14 onto the polarizingbeamsplitter 16. The light polarized by the polarizing beamsplitter 16is reflected onto the display engine 14, which can preferably be an LCoSdisplay, to produce polarized, spatially and temporally modulated lightwhich defines the synthetic image. The synthetic image is relayed by thedisplay optics 72 to the wire grid polarizing combiner 20 which reflectsthe image toward the pupil 25 of the eye 24 of the wearer of the HMD 70.The wire-grid polarizing combiner 20 reflects the synthetic image intothe eye 24, while allowing light and images from the outside world,i.e., real images 22, to also be contemporaneously imaged by the eye 24,thus combining, or overlaying, the synthetic image onto the real image22.

Some differences are that, in the HMD 70, the illumination optics 74 canbe an optical device commonly called a Fresnel lens, and the displayoptics 72 can comprise image enlargement optics (a magnifier) commonlycalled a modified Ploessl arrangement. The modified Ploessl arrangementcomprises two identical lenses 76, 78. In the arrangement illustrated,each lens is a two element doublet 76 a, 76 b and 78 a, 78 b. Themodified Ploessl configuration may be chosen because of the vast amountof available stock optics which can be used to implement thearrangement. The modified Ploessl configuration also allows for a sortfocal length, long eye relief and a relatively large field of view.Ploessl configurations are also characteristically insensitive to pupilshifts, which allowing a larger eye-motion box.

Moreover, an additional contrast enhancing polarizing plate 80, alsotermed a clean-up polarizer, can optionally be utilized between thepolarizing beamsplitter 16 and the display optics 72 to eliminateunpolarized or partially polarized rays. Generally, the clean uppolarizer 80 would only be used where poorer quality polarizers areused, or when extremely high light levels are used, and the extinctionratio needs to be increased. Additionally, there may be some cases inwhich a fiber optic face plate 82 could be used over the LCoS display toeliminate skew rays.

Although certain embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications to those details could be developed in light of theoverall teaching of this disclosure. Accordingly, the particularembodiments disclosed herein are intended to be illustrative only andnot limiting to the scope of the invention which should be awarded thefull breadth of the following claims and any and all embodimentsthereof.

1-56. (canceled)
 57. A method of combining a synthetic image with acontemporaneously viewable real image in a wearable display device, saidmethod comprising: a. producing light representative of a syntheticimage; b. relaying said light to a wire grid polarizing combiner; c.reflecting at least a portion of said light toward said polarizedcombiner, said reflected portion of light representative of saidsynthetic image; and d. contemporaneously imaging said real imagethrough said wire grid polarizing combiner, such that said syntheticimage is overlaid onto said real image.
 58. The method of claim 57further comprising: a. linearly polarizing said light; and b.substantially entirely reflecting said linearly polarized light at saidwire grid polarizing combiner.
 59. The method of claim 58 furthercomprising curving in a single axis said wire grid polarizer.
 60. Themethod of claim 58 further comprising curving in two axes said wire gridpolarizer.
 61. The method of claim 60 further comprising: a. enlargingsaid synthetic image; and b. correcting image aberrations caused due tosaid wire grid polarizing combiner being curved in two axes.